1. Technical Field
The present invention relates generally to electronic circuits and in particular to transmitter circuits. Still more particularly, the present invention relates to serial link transmitter circuits and design thereof.
2. Description of the Related Art
The ability to perform and achieve high speed transmissions of digital data has become expected in today's computing environment. In most cases, the transmission of digital data over longer distances is accomplished by sending the data in a high-speed serial format (i.e., one single bit after another) over a communication link designed to handle computer communications. In this fashion, data can be transferred from one computer system to another, even if the computer systems are geographically remote.
In order for high-speed serial transmission to occur, the digital data signal from inside the computer must be transformed from the parallel format into a serial format prior to transmission of the data over the serial communication link. This transformation is generally accomplished by processing the computer's internal data signal through electronic circuitry known as a serial link transmitter or “serializer.” The function of the serializer is to receive a parallel data stream as input and, by manipulating the parallel data stream, output a serial form of the data capable of high-speed transmission over a suitable communication link. Once the serialized data has arrived at the desired destination, a piece of computer equipment known as a “deserializer” is employed to convert the incoming data from the serial format to a parallel format for use within the destination computer system.
Conventional high speed serial link transmitters are typically implemented using current-mode circuit techniques and thus require a parallel termination at the source end. When a parallel source end termination is utilized, however, only half of the transmitter output current is passed to the receiver as the transmitter steers the current through only one of the two parallel sides of the transmitter output to the receiver. Thus, to produce a desired output amplitude (e.g., voltage), the current-mode transmitter with parallel termination requires a relatively larger power dissipation to adjust for the loss of approximately 50% of the current.
One development that eliminates this large power dissipation requirement in providing the desired output amplitude is the introduction of a self-series terminated transmitter. With a self-series terminated transmitter, substantially all the transmitter's output current goes to the receiver. Thus, for a given signal amplitude at the receiver, a self-series terminated transmitter dissipates significantly less power than a current-mode transmitter with parallel termination, because all of the self-series terminated transmitter output current goes to the receiver.
Several types of self-series terminated transmitters have been described in prior art. While the self-series termination transmitters provide more efficient power usage, several other limitations have been noted with conventional designs, including ones related to output amplitude control, pre-emphasis control, and slew rate control, among others. Thus, as data transmission speeds continue to increase, there exists an ongoing need for an even better transmitter.